DO you know for sure that you have a 27.0mm crown race?

Yes you can do it, it's best to have a fork clamp so you can rotate the fork and do a nice even job with the file. Some people will throw up their arms and say this is lunacy but I have done it more than once, these are bicycles and not rocket science so if you are careful and smooth you can do a nice job, I have cleaned up other peoples horrific jobs also with good results. However, I do have a very nice sealed specialized headset in JIS (27mm crown race if you are interested PM me).